FAQ-Wiki-Outlook

Why is the size of a converted PST file smaller than the size of the original OST file?

Answer:

Our engineers have analyzed the content of OST (source) and PST (destination) files. The analysis revealed the types
of data that are not carried over to the PST file after OST file conversion:

old versions of messages, contacts, files and other objects

removed objects

blocks of data from earlier versions of messages, contacts, files and other objects. It is impossible to use these
blocks to fully recreate the original object, which is why they can be treated as junk.

In other words, when you remove or modify a message, contact, reminder or meeting, the object is only marked as removed,
but is not removed physically and remains in the OST file. The new version of the object is written to a newly allocated
space, but the old version remains in the OST file as well.

If Outlook is in active use, the size of the OST file can grow quite significantly, while the actual size of data
in the file remains the same.

In order to optimize disk space consumption and remove old elements, you need to configure the AutoArchive feature
in Microsoft Outlook.

Note:

search for and reading of old versions of documents is used by attackers for stealing users’ confidential information;

search for and reading of deleted and old versions of documents is used by police authorities to collect compromising
information (Forensic Mode);

Recovery Toolbox for Outlook enables the user to search for data deleted from an OST/PST file and save
it to the disk.